Sphingosine (SPN) is a long chain unsaturated amino alcohol of the formula C.sub.18 H.sub.37 O.sub.2 N found in cell membranes and in high concentration in nervous tissue. Sphingosine and sphingoid base (a long chain aliphatic base comprising a 1,3-dehydroxy-2-amino group at a terminus and derivatives thereof) have been implicated as inhibitors of protein kinase-C (PK-C) and EGF receptor-associated tyrosine kinase (EGF-RK) (Hannun & Bell, Science, 235, 670, 1987; Hannun, JBC, 261, 12604, 1986; Kreutter et al., JBC, 262, 1632, 1987).
Protein kinase-C activity is related closely to cell growth and recent studies indicate that increased tumorigenicity is correlated with over expression of PK-C.sub..beta.1 and PK-C.sub..gamma. in some experimental tumors (Housey et al., Cell, 52, 343, 1988; Persons et al., Cell, 52, 447, 1988). A mutant PK-C.sub..alpha. induces highly malignant tumor cells with increased metastatic potential (Megidish & Mazurek, Nature, 324, 807, 1989). It would appear that aberrant expression of PK-C may relate to tumor progression.
Recent studies indicate that phospholipids, sphingolipids and metabolic products thereof have an important role in the modulation of transmembrane signaling through PK-C and other membrane-associated kinases, such as EGF receptor-associated tyrosine kinase (Hakomori, JBC, 265, 18713, 1990). For example, PK-C activity is promoted by diacyl glycerol and inhibited by sphingosine (Hannun & Bell, supra; Hannun & Bell, Science, 243, 500, 1989; Merrill & Stevens, Biochem. Biophys. Acta, 1010, 131, 1989).
Sphingosine did not inhibit PK-C in vitro or at concentrations below 100 .mu.M and did not exhibit a stereospecific effect on PK-C (Igarashi et al., Biochem., 28, 6796, 1989). Many of the studies described above employed sphingosine obtained from a commercial source (for example Sigma Chemical Company) and the preparations contained various impurities including 3-O-methylsphingosine, 5-O-methylsphingosine and N-methylsphingosine. The impurities are likely to result from the process of preparation, namely methanolysis of sphingomyelin or cerebroside. Furthermore, in the sphingosine backbone, the D-erythro configuration about the chiral carbons is often converted to the D-threo configuration.
Igarashi et al. (supra) found that the inhibitory effect of sphingosine on PK-C activity is due to: (1) the stereospecific configuration of C1 to C3 (D-erythro configuration required); (2) presence of a long-chain aliphatic group; and (3) perhaps most essential, a negative charge at the primary amino group at C2. If the amino group was N-acetylated, the PK-C inhibitory activity was abolished since the negative charge of an amino group was eliminated by acetylation. However if the anionic character of the amino group was enhanced by N-methylation, the stereospecific PK-C inhibitory activity was enhanced.
Interaction of leukocytes with activated platelets and endothelial cells is an initial step in inflammatory processes and is mediated in part by a family of adhesion molecules known as selectins. Selectins include MEL-14 in mouse and ELAM-1, LAM-1 and GMP-140 (CD62/PADGEM) in man. The selectins are characterized by a similar structural motif consisting of a lectin domain at the N-terminal region, followed by an epidermal growth factor sequence, a complement-regulatory domain, a transmembrane region and a C-terminal domain (Stoolman, Cell, 56, 907, 1989 and Osborn, Cell, 62, 3, 1990). Members of the selectin family bind carbohydrate ligands (see for example, Springer, Nature, 346, 425, 1990; Brandley et al., Cell 63, 861, 1990; Lowe et al., Cell, 63, 475, 1990; and Walz et al., Science, 250, 1132, 1990).
Based on inhibition studies using a variety of glycosphingolipid liposomes, the binding epitopes of both ELAM-1 and GMP-140 expressed on HL60 (a human promyelocytic cell line) cells were identified as sialosyl-Le.sup.x (Phillips et al., Science, 250, 1130, 1990 and Polley et al., Proc. Natl. Acad. Sci., 1991, in press).
Expression of selectins is up-regulated by the inductive effect of lymphokines, tumor necrosis factor (TNF.alpha.), bacterial lipopolysaccharides phorbol esters, thrombin and perhaps many other compounds. Leukocytes, together with platelets, thereby are recruited to the inflammatory site. Since tumor cells are capable of activating platelets (see for example, Ugen et al., J. Natl. Canc. Inst., 80, 1461, 1988; Watanabe et al., Canc. Res., 48, 6411, 1988; and Grignani & Jamieson, Blood, 71, 844, 1988), a similar process can be expected to occur during tumor cell adhesion on microvascular endothelia. Thus, the process of tumor cell metastasis may be initiated by selectindependent tumor cell adhesion Although there is no evidence of direct activation of endothelial cells by tumor cells, IL-1 or TNF.alpha.-activated endothelial cells have been shown to adhere to a variety of tumor cells (Walz et al. supra).
While the regulatory mechanism for expression of selectins is understood poorly, it apparently involves a complex sequence of transmembrane signaling transducers including protein kinase-C, members of the G-protein family (for example, ras, G.sub.s, G.sub.i, G.sub.0 etc.) and a 47 kDa phosphoprotein, all of which have been shown to be modulated by glycosphingolipids and sphingosine derivatives. Platelet aggregation and associated ATP secretion are inhibited strongly by trimethylsphingosine (TMS). The phenomenon could result from inhibition of 47 kDa protein phosphorylation or of phosphoinositide turnover as a membrane signaling pathway in platelets.